The present invention relates to automated equipment used in dispensing viscous materials. More particularly, the present invention relates to a system and method which in their preferred embodiments may be used for automatically dispensing the underfill epoxy used to attach silicon chips directly to printed circuit (xe2x80x9cPCxe2x80x9d) boards comprising FR4 or similar laminate substrates.
In the manufacture of PC boards it is frequently necessary to apply small amounts of viscous materials, i.e. those with a viscosity greater than fifty centipoise. Such materials include, by way of example and not by limitation, general purpose adhesives, solder paste, solder flux, solder mask, grease, oil, encapsulants, potting compounds, epoxies, die attach pastes, silicones, RTV and cyanoacrylates. Heretofore the common methods of application have included screening, pin transfer and dispensing from a syringe or valve. Screening requires a template and is not readily adaptable to changing application patterns. Pin transfer is relatively fast but the tooling is expensive and inflexible and can only form dots, not lines. Syringe dispensing is widely used and is accomplished with pneumatic mechanisms, electromechanical mechanisms or positive displacement valves.
In the quest for ever increasing miniaturization of circuitry a fabrication process known as flip chip technology has developed. This technology is also known as direct chip attach or xe2x80x9cDCAxe2x80x9d. It includes xe2x80x9cflip chipxe2x80x9d bonding, dies attached directly to substrates, wire bonding, coated dies and encapsulated dies. One such process which is widely used is called controlled columnar collapsed connection (xe2x80x9cC4xe2x80x9d) which is covered by U.S. Patents owned by International Business Machines Corporation.
Referring to the drawings, a semiconductor die-or flip chip 10 (FIG. 1) is provided with a pattern of solder bumps or balls 12 on an underside or circuit side thereof The solder balls 12 are registered with plated solder pads 14 on a PC board or other substrate 16. The underside of the chip 10 is also referred to as the image side of the chip. Flux (not illustrated) is normally supplied between the solder balls 12 and solder pads 14. Upon heating, the solder pads 14 on the PC board or substrate 16 reflow and physically connect with the solder balls 12 on the underside of the chip 10. The solder balls 12 typically have a high melting point and therefore do not reflow. This connection is illustrated diagrammatically in FIG. 2 by deformed solder pad 14xe2x80x2 mating with a solder ball 12. The requirement for wire bonding is thereby eliminated.
Since the flip chip 10 is not necessarily encapsulated in a plastic or ceramic package, the connections between the PC board 16 and the chip 10 can corrode. In order to prevent this corrosion, a special liquid epoxy 18 (FIG. 3) is allowed to flow and completely cover the underside of the chip. This is referred to herein as the xe2x80x9cunderfill operation.xe2x80x9d Upon curing, the resulting encapsulation forms a non-hygroscopic barrier to prevent moisture from contacting and thus corroding the electrical interconnects between the PC board 16 and the chip 10. The epoxy 18 also serves to protect the bonds between the deformed solder pads 14xe2x80x2 and the solder balls 12 by providing thermal stress relief, i.e. accommodating differential rates of thermal expansion and contraction. Stated another way, once cured the epoxy 18 has a co-efficient of thermal expansion (xe2x80x9cCTExe2x80x9d) which together with its bonding properties minimizes the thermal stress induced by the difference between the CTE of the silicon chip 10 and the CTE of the FR4 PC board 16.
Advantages of using flip chip on board architecture include: 1) the potential for increased input and output (xe2x80x9cI/Oxe2x80x9d) as the entire die area beneath the chip is available for connection; 2) an increase in electronic processing speed due to shorter transmission line lengths; 3) the ability to fit a heat sink to the top of the chip, 4) a substantial reduction in chip profile and 5) more efficient use of PC board real estate.
Referring to FIG. 3 of the drawings, once the underfill operation is complete, it is desirable that enough liquid epoxy be deposited to encapsulate all of the electrical interconnections and so that a fillet 18a is formed along the side edges of the chip 10. A properly formed fillet 18a ensures that enough epoxy has been deposited to provide maximum mechanical strength of the bond between the chip 10 and the PC board or substrate 16. If too much epoxy is deposited, a mound 18b (FIG. 4) will be produced which undesirably encircles the side edges of the chip 10 and extends along the upper surface of the chip.
The aforementioned underfill operation requires that a precise amount of the liquid epoxy 18 be deposited in a more or less continuous manner along at least one side edge of the semiconductor chip 10. The liquid epoxy flows under the chip 10 as a result of capillary action due to the small gap between the underside of the chip 10 and the upper surface of the PC board or substrate 16. If too little epoxy is deposited, some of the electrical interconnections will not be encapsulated. Corrosion may result and thermal stresses may not be relieved. If too much epoxy is deposited, it may flow beyond the underside of the chip and interfere with other semiconductor devices and interconnections. Excess epoxy may also encroach on the upper side of the chip 10 as shown at 18b in FIG. 4 and interfere with proper heat dissipation of a heatsink.
During the underfill operation, it is necessary to precisely control the temperature of the liquid epoxy or other liquid adhesive. The liquids that are utilized are often stored in a frozen state. They are thereafter thawed and utilized in connection with a dispensing syringe. However the viscosity of this type of adhesive changes rapidly with time as it cures, sometimes doubling within four hours of being thawed. This complicates the task of dispensing the correct amount of fluid, because if its viscosity increases too much, capillary action will not be sufficient to make it completely flow under the chip. Therefore, there is a need to determine when the liquid adhesive has reached the predetermined viscosity that renders it no-useable in an underfill operation.
In the past PC boards have been heated by conduction through direct mechanical contact, with lamps or with convective heat, i.e. gas flow. Often such heating is performed in belt ovens having successive air zones, the temperature of which can be independently controlled to achieve a given heating profile. It has also been conventional in the dispensing of minute amounts of adhesives and other viscous materials to employ a heater for maintaining the temperature of the dispensing needle and/or valve, and the viscous material therein, at a predetermined level. However, the prior art methods of controlling temperature in the conventional assembly of PC boards have not provided very accurate control of the viscosity.
Accordingly, it would be desirable to provide an automated viscous material dispensing system and method in which the amount of material dispensed could be precisely controlled, taking into account variations in the viscosity of the material itself.
The present invention provides a system for dispensing a viscous material onto a substrate which includes a dispensing element, a viscous material reservoir and a metering device coupled between the reservoir and the dispensing element for metering a variable amount of a viscous material through the dispensing element. The dispensing element and metering device can be moved by a positioner along a predetermined pattern adjacent a surface of a substrate. A weigh scale located adjacent the substrate receives a metered amount of the viscous material and produces signals representative of a variable weight of the material dispensed during a predetermined time interval. Thus a flow rate of material can be accurately determined. A controller adjusts a speed of movement of the positioner along the predetermined pattern to cause the dispensing element to dispense a desired amount of material based on a calculated flow rate. Alternatively, the controller adjusts a rate of delivery of the metering device based on the calculated flow rate to cause the dispensing element to dispense the desired amount of material along the predetermined pattern. Closed loop temperature control may be provided for the dispensing element and/or the substrate to ensure a substantially constant viscosity and a substantially constant flow rate. A prime and purge station may be provided adjacent the substrate for sucking air bubbles from the dispensing element and metering device.